Uncaught TypeError: Cannot call method 'indexOf' of undefined

Let me preface all of this that I'm an ExtJS & Sencha Touch newbie. I come from jQuery, so this is all pretty new to me.

I'm attempting to create a Nested List using data from a JSONP source. I can see that I'm getting data (via console.log), but it's erring out "Uncaught TypeError: Cannot call method 'indexOf' of undefined". I'd love to hear what I'm doing wrong.